1. Field of the Invention
The present invention relates to a valve timing control device for an internal combustion engine, the device comprising a variable valve timing mechanism for controlling the valve overlap period.
2. Description of the Related Art
When the valve overlap period, during which the intake valve and the exhaust valve are opened simultaneously at the end of an exhaust stroke, is made long, the trapping efficiency and the scavenging efficiency increase so that a good performance at high engine speed can be obtained. On the other hand, in the idle condition, the long valve overlap period causes a back-flow of a large amount of exhaust gas so that combustion deteriorates. Accordingly, in an idle condition, it is desirable that the valve overlap period is made short. For this purpose, a variable valve timing mechanism capable of varying the valve overlap period has already been suggested.
A known valve timing control device for controlling the valve overlap period by means of the variable valve timing mechanism determines an optimal valve overlap period in accordance with a current engine operating condition on the basis of current engine load and engine speed. Moreover, Japanese Unexamined Patent Publication No. 4-194331 discloses a valve timing control device which makes the optimal valve overlap period determined for each engine operating condition shorter when the engine has not warmed-up because combustion at this time is unstable in contrast with when the engine is warmed-up.
According to the above valve timing control device, in each engine operating condition when the engine has not warmed up, the amount of back-flow exhaust gas is reduced so that combustion can become stable. However, in high engine load operating conditions, when the engine has not warm up, the required engine torque cannot be obtained.